This invention relates to semiconductor vertical-cavity surface-emitting lasers (VCSELs), and more particularly to structures and techniques for providing highly-efficient and single-mode VCSELs. A VCSEL is a semiconductor laser consisting of a semiconductor layer of optically active material, such as gallium arsenide or indium gallium arsenide or the like, sandwiched between highly-reflective layers of metallic material, dielectric material, epitaxially-grown semiconductor dielectric material or combinations thereof, most frequently in stacks known as distributed Bragg reflectors. As is conventional, one of the mirror stacks is partially reflective so as to pass a portion of the coherent light built up in the resonating cavity formed by the mirror stack/active layer sandwich.
Laser structures always require optical confinement and carrier confinement to achieve efficient conversion of pumping electrons to stimulated photons. Carrier confinement is generally achieved by varying the resistivity of the materials between the electrical contacts and the active region. Such techniques include introduction of high resistivity through ion bombardment, variations in doping and dopant type, removal of conductive material by etching as well as conversion of semiconductor into insulating oxide by selective oxidation. Optical confinement is achieved by varying the index of refraction of the materials in the structure. It is convenient to discuss the cavity in terms of a cylindrical geometry, although many cross sections are possible and the invention is not limited to cylindrical geometries. The axial mode's standing-wave pattern in a VCSEL is very strong due to the high reflectivity of the mirrors, typically in excess of 99%. The relatively short optical path length between the two mirrors results in a relatively large wavelength separation between resonant axial modes. The large wavelength separation ensures that the VCSEL lases in only a single axial mode. The transverse or radial optical mode's intensity profile is determined by radial index variations in the cavity. A desirable mode is the fundamental mode, for example the HE.sub.11 mode of a cylindrical waveguide. A fundamental mode signal from a VCSEL is easy to couple into an optical fiber, has low divergence, maintains a stable far field pattern and is inherently single frequency in operation.
VCSELs which introduce radial index variations for optical confinement are known as index-guided VCSELs. In a conventional edge-emitting semiconductor laser, the transverse index guide (perpendicular to the growth direction) is designed for an index variation on the order of 0.1% to achieve single transverse mode operation. The small index variation precludes the existence of higher order transverse modes. Regrown or ridge waveguide structures are typically used in edge-emitting lasers to form the transverse index guide. In order for a VCSEL to lase, the mirror reflectivities in the axial direction must be very high and thus the layers must be epitaxially grown or deposited with a high degree of planarity. The requirement of high planarity has precluded the effective use of regrowth or ridge waveguide designs in VCSELs.
The previously-known index-guided VCSELs use either etched-post or insulating slots to provide carrier confinement and introduce a radial index variation. Unfortunately, the radial index variation is relatively large and results in a cavity which supports multiple transverse modes. Furthermore, the carrier confinement is generally at a diameter equal to or larger than the transverse optical-mode diameter. As the optical mode is weak at the edges, the of conversion of carriers into light by stimulated emission is reduced substantially for those carriers in the active region outside the characteristic diameter (or beam waist) of the transverse optical mode.
What is needed is a VCSEL structure that provides carrier confinement to a diameter less than that of the transverse optical mode and yet can be realized while maintaining a high degree of planarity in the layers. In co-assigned U.S. Pat. No. 5,343,487, the inventors disclosed a VCSEL that provided a constriction of carriers to an aperture less than the transverse optical mode. The disclosed laser used a ring-contact geometry so that ion implantation or diffusion techniques could be used to alter the conductivity of a semiconductor layer, providing a current constriction without introducing an optical constriction. The earlier patent also disclosed a resistive current-leveling layer between the active region and the conductive layer to minimize current-crowding effects associated with ring-contacted junctions. The drawbacks of this laser are the complexity of fabricating a ring-contacted geometry and the difficulties of using the semiconductor altering techniques, such as ion implantation and diffusion, that do not introduce significant refractive-index discontinuities.
It would be highly desirable to be able to introduce a current constriction with technologically-simpler techniques that introduce refractive-index discontinuities without constricting the transverse optical mode as well. The present invention allows such simpler techniques for current construction to be used without constricting the optical mode, and therefore the present invention has broad applicability to a wide variety of VCSEL structures.